The present invention relates to a multi-beam scanning optical system in which a plurality of light beams emitted by a plurality of light sources are deflected to scan on a surface to be scanned such as a circumferential surface of a photoconductive drum.
A scanning optical system to be employed in a laser printer for forming a monochrome (e.g., a black-and-white) image is provided with a laser diode, which is driven in accordance with image data. The laser beam emitted by the laser diode is collimated by a collimating lens, and is directed to a deflecting member such as a polygonal mirror. The laser beam, which is incident on light reflecting surfaces of the rotating polygonal mirror, is deflected to scan within a predetermined angular range. The scanning laser beam is incident on an fxcex8 lens, refracted and converged thereby, and then is incident on an evenly charged photoconductive surface of a photoconductive drum to form a beam spot which moves along the rotational axis of the photoconductive drum (i.e., along a main scanning direction). Since the laser diode is driven in accordance with the image data, the surface of the photoconductive drum is exposed to light corresponding to the image data. While the light beam scans on the surface of the photoconductive drum, it is rotated (i.e., an auxiliary scanning is performed). Thus, a two-dimensional latent image is formed on the photoconductive surface of the photoconductive drum.
Then, toner is applied to the latent image to form a developed image, which is transferred onto a recording sheet and fixed thereon.
Recently, color laser beam printers, which are capable of forming color images, have been developed. In the color laser printer, generally, a plurality of laser diodes are provided (which will be referred to as a multi-beam laser printer). Further, the corresponding number of fxcex8 lenses, and the corresponding number of photoconductive drums are provided for forming images of respective color components (e.g., yellow, magenta, cyan and black components). The above-described, exposing and developing processes are performed for each color component, and thus formed color images (developed image) for the four color components are transferred on a recording sheet and fixed.
In the multi-beam laser printer as described above, since a plurality of fxcex8 lenses are employed for respective color components, the problems indicated below occur.
Firstly, various units such as an exposing unit, developing unit, and transferring unit, for performing an electro-photographic imaging process should be arranged around each photoconductive drum, and therefore a space for arranging such units should be provided. However, if the fxcex8 lens includes a plurality of lenses, which occupy the space and it becomes difficult to maintain flexibility in designing the optical paths within the scanning optical device.
Further, since, in the conventional scanning optical system, the number of elements constituting the fxcex8 lens is large, and therefore the manufacturing cost is raised.
In view of the above problems, an object of the present invention is to provide an improved scanning optical system in which the space occupied by the fxcex8 lens is reduced to increase flexibility in designing the optical paths. Another object of the present invention is to provide an improved scanning optical system in which the number of elements is reduced to reduce the manufacturing cost thereof.
For the above objects, according to one aspect of the invention, there is provided a multi-beam scanning optical system, which is provided with a light source unit emitting a plurality of beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and reflected by the reflection surfaces of the polygonal mirror, and an fxcex8 lens system, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the fxcex8 lens system and proceeding toward surfaces to be scanned. In the above structure, the fxcex8 lens includes a first lens that converges the beams mainly in the main scanning direction, and a second lens that converges the beams mainly in the auxiliary scanning direction. The beams reflected by the polygonal mirror pass through the first and second lenses in this order. The first and second lenses are made of different materials, one of the first and second lenses being formed integrally with the other using a mold such that the first and second lenses form a composite lens.
Since the first and second lenses included in the fxcex8 lens system are integrally formed as a composite lens unit, the space occupied by the fxcex8 lens is reduced. Further, since the two lens elements are integrally formed to one element, the number of elements is also reduced.
Optionally, the second lens is provided with convex surfaces, on a light emerging surface of the second lens, extending in the main scanning direction and converging the beams in the auxiliary scanning direction.
Further optionally, diffraction lens structures can be provided on a light receiving surface of the first lens. In this case, the diffraction lens structure may be formed on a film adhered onto the light receiving surface of the first lens. Alternatively, the diffraction lens structure may be integrally formed on the light receiving surface of the first lens.
Alternatively, the diffraction lens structures can be provided on a light emerging surface of the second lens, the diffraction lens structures being formed on the convex surfaces, respectively.
Still optionally, the diffraction lens structures may be formed on the light receiving surface of said first lens at positions where the beams incident, respectively. Alternatively, diffraction lens structures may be formed on the light emerging surface of said second lens at positions where the beams incident, respectively.
According to another aspect of the invention, there is provided a multi-beam scanning optical system, which is provided with a light source unit emitting a plurality of beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and reflected by the reflection surfaces of the polygonal mirror, and an fxcex8 lens system, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the fxcex8 lens system and proceeding toward surfaces to be scanned. In this case, the fxcex8 lens may include a first lens that converges the beams mainly in the auxiliary scanning direction, and a second lens that converges the beams mainly in the main scanning direction. The beams reflected by the polygonal mirror pass through the first and second lenses in this order. Further, the first and second lenses are made of different materials, one of the first and second lenses being formed integrally with the other using a mold such that the first and second lenses constitute a composite lens.
Optionally, the first lens is provided with convex surfaces on a light receiving surface of the first lens, the convex surfaces extending in the main scanning direction and converging the beams in the auxiliary scanning direction.
Further optionally, diffraction lens structures are formed on the light receiving surface of the first lens at positions where the beams incident, respectively.
Alternatively, diffraction lens structures may be formed on the light emerging surface of the second lens at positions where the beams incident, respectively.
In one particular case, diffraction lens structures are provided on a light receiving surface of the first lens, the diffraction lens structures being formed on the convex surfaces, respectively.
In another particular case, diffraction lens structures are provided on a light emerging surface of the second lens. In this case, the diffraction lens structure may be formed on a film adhered onto the light emerging surface of the second lens. Alternatively, the diffraction lens structure may be integrally formed on the light emerging surface of the second lens.